A set is a collection of values. Although the abstract concept of a set does not necessarily imply an ordered collection, the standard library set datatype is always ordered.
NOTE -- If necessary, a collection of values that cannot be ordered can be maintained in an alternative data structure, such as a list.
Because the container used to implement the set data structure maintains values in an ordered representation, sets are optimized for insertion and removal of elements, and for testing to see whether a particular value is contained in the collection. Each of these operations can be performed in a logarithmic number of steps, whereas for a list, vector, or deque, each operation requires in the worst case an examination of every element held by the container. For this reason, std::set should be the data structure of choice in any problem that emphasizes insertion, removal, and test for inclusion of values. Like a list, a set is not limited in size, but rather expands and contracts as elements are added to or removed from the collection.
There are two varieties of sets provided by the C++ Standard Library. In the set container, every element is unique, and insertions of values that are already contained in the set are ignored. In the multiset container, on the other hand, multiple occurrences of the same value are permitted.
NOTE -- In other programming languages, a multiset is sometimes referred to as a bag.
Whenever you use a set or a multiset, you must include the set header file: